This application claims the priority of Korean Patent Application No. 2003-58841, filed on Aug. 25, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a nitride-based light emitting device and a method of manufacturing the same, and more particularly, to a nitride-based light emitting device having an Ohmic contact structure for increasing a quantum efficiency, and a method of manufacturing the same.
2. Description of the Related Art
In order to embody a light emitting device such as a light emitting diode or a laser diode by using a nitride-based compound semiconductor, for example, a gallium nitride (GaN) semiconductor, Ohmic contact structure between a semiconductor and an electrode is of much importance. A gallium nitride-based light emitting device is formed on an insulating sapphire (Al2O3) substrate.
The gallium nitride-based light emitting device is classified into Top-Emitting Light Emitting Diodes (TLEDs) and Flip-Chip Light Emitting Diodes (FCLEDs).
The top-emitting light emitting diode allows light to emit through an Ohmic electrode layer that is in contact with a P-type clad layer, and provides a low electric conductivity of the P-type clad to allow smooth current injection through the Ohmic electrode layer with transparency and low resistance.
The top-emitting light emitting diode is generally employing a structure of a nickel (Ni) layer and a gold (Au) layer sequential layered on the P-type clad layer.
The nickel layer is known in the art to form a semi-transparent Ohmic contact layer that is annealed in oxygen (O2) atmosphere to have a relative contact resistance of about 10−3-10−4 Ωcm2.
When the semi-transparent Ohmic contact layer is annealed at about 500-600° C. in the oxygen atmosphere, the semi-transparent Ohmic contact layer provide a low relative contact resistance between the gold (Au) layer and a lower layer portion where the nickel oxide (NiO) is island-shaped as a P-type semiconductor oxide between the gallium nitride that forms the P-type clad layer and the nickel layer that is used as the Ohmic contact layer. Accordingly, a Schottky Barrier Height (SBH) is reduced, thereby facilitate to supply holes as a majority carrier in the vicinity of a surface of the P-type clad layer. As a result, an effective carrier concentration is increased in the vicinity of the surface of the P-type clad layer.
Further, after the nickel layer and the gold layer are formed on the P-type clad layer, a reactivation process using the annealing is performed to remove a Mg—H compound to thereby increase a concentration of Magnesium dopants at a surface of the gallium nitride. As a result, the effective carrier concentration of above 1019 is obtained at the surface of the P-type clad layer. Therefore, tunneling conduction is generated between the P-type clad layer and the Ohmic electrode layer that contains nickel oxide to provide an Ohmic conduction characteristic.
However, since the top-emitting light emitting diode using a semi-transparent electrode film formed of nickel/gold has a low optic efficiency, it is difficult to embody a large-capacity and high-luminance light emitting device.
In order to embody the large-capacity and high-luminance light emitting device, a flip-chip light emitting device using silver (Ag) or aluminum (Al) that is noticed as a high reflective material is being recently required for development.
However, silver or aluminum can temporarily provide a high light-emitting efficiency due to its high reflection efficiency, but there is a drawback in that a device life is short since it is difficult to form an Ohmic contact with a lower resistance due to a small work function, and a stable device reliability is not provided since the adhesiveness with the gallium nitride is poor.
In order to solve the above drawback, an Ohmic contact layer providing the high reflectivity despite the low relative contact resistance is being vigorously studied for development.
U.S. Patent Publication No.: 2002-0190260A1 discloses a structure with nickel/silver sequential layered on the P-type clad layer, but has a drawback in that contact resistance is high, and adhesiveness is low at the time of annealing.